1. Technical Field
Embodiment of the invention relates to an optical information reproducing apparatus, and more particularly, to an optical information reproducing apparatus and an optical information reproducing method, which can detect a tilt error using a reference beam and a pair of assistant beams and correct the detected tilt error.
2. Background Art
As an optical data processing apparatus, a digital versatile disc (DVD), a high-definition DVD (HD-DVD), a blu-ray disc (BD), a near field optical information processing apparatus, and a holographic optical information processing apparatus are known.
The holographic optical information processing apparatus records data by irradiating a signal beam which is optically modulated and a reference beam which serves to form an interference pattern in a recording medium, to the recording medium to intersect each other.
The holographic optical information processing apparatus reproduces the recorded data by irradiating only a reference beam to the recording medium, detecting a reproduction beam reproduced from the recording medium by the use of a detector, and processing the detecting reproduction beam.
The holographic optical information processing apparatus can repeatedly record data at the same position of an optical information recording medium in a three-dimensional manner by the use of a variety of multiplexing methods. Accordingly, it is possible to markedly enhance the storage capacity of an optical information recording medium having a limited area. Examples of multiplexing methods can include an angle multiplexing method, a phase-code multiplexing method, a wavelength multiplexing method, a fractal multiplexing method, a shift multiplexing method, and a peristrophic multiplexing method.
Optical information may not be stored only at a position on the storage medium. That is, a variety of optical information can be recorded at a variety of positions on a recording medium. Accordingly, the recording medium should be moved to a position for reproducing optical information to reproduce the optical information stored in the recording medium. For example, when the recording medium is of a disc type, the optical information is reproduced while the disc is rotated.
A method of moving the recording medium is described in U.S. Pat. No. 5,978,112, entitled “NON-VOLATILE READOUT OF SHIFT MULTIPLEXED HOLOGRAMS” and filed by Psaltis et al.
The US patent discloses a structure and a method of tilting a recording medium. That is, the recording medium is allowed to move using a shift servo, depending on a recording position of optical information on the recording medium at the time of recording the optical information.
As described in the U.S. patent, the recording medium which continuously moves to reproduce optical information necessarily accompanies a mechanical motion such as a rotational motion and a translational motion. Accordingly, the tilt angles of the reference beam and the recording medium can be continuously varied.
When a volume hologram is used like the U.S. patent, the reference beam should be incident on a volume holographic area having a large angle selectivity to enhance reproduction efficiency of a reproduction beam.
When the incident angle of a reference beam incident on the recording medium storing optical information is in the range of Brag angle, the reproduction efficiency of a reproduction beam reproduced in response to the reference beam is enhanced.
The incident angle of the reference beam is determined in accordance with the tilt angle of the recording medium. A diffraction efficiency of the reference beam incident on the recording medium depends on the incident angle of the reference beam (that is, the tilt angle of the recording medium). When the incident angle of the reference beam varying depending on the tilt angle of the recording medium departs from the range of Brag angle, the reproduction efficiency of the reproduction beam is markedly reduced.
Accordingly, it is necessary to detect whether a tilt error occurs between the reference beam and the recording medium and to correct the tilt error, at the time of recording data in the recording medium or reproducing data from the recording medium.